Worlds collide when Barry uses his superpowers to travel back in time in order to change the events of the past. But when his attempt to save his family inadvertently alters the future, Barry becomes trapped in a reality in which General Zod has returned, threatening annihilation, and there are no Superheroes to turn to. That is, unless Barry can coax a very different Batman out of retirement and rescue an imprisoned Kryptonian, albeit not the one...

"If you could go back in time and alter events in your life, would you do it? Would you change your childhood? Bring back a loved one from the grave? Of course you would. Or would you? Would you be prepared for a string of terrifying unintended consequences that come from playing God?"--Case.